The present invention concerns a novel method for mechanical disintegration of elastic materials and in particular vulcanised rubber, an intermediate composite body used in said method and the powder produced by said method.
Both environmental and economic considerations make the reuse of materials an important issue. Different waste materials are sorted, subjected to various treatments and used as raw materials for the manufacture of new products. In many cases the properties of the waste makes it difficult to process the material into a form suitable for reuse. Such properties can be the physical properties of the waste, such as its elasticity. One example of such materials is vulcanised rubber, the material of car tyres, conveyor bands, rubber mats, etc.
Worn out tyres are associated with special problems, both regarding recycling and disposal on waste disposal or landfill sites. In addition to containing a rubber matrix comprising natural and/or synthetic rubber, modern tyres also contain carbon black, plasticizers, cross-linking agents, anti-oxidants, anti-ozone agents and other performance improving additives plus a reinforcement structure, consisting of metal wire or fibres.
In additional to taking up a great deal of space on waste disposal sites, the dumping of tyres cause several other problems as well. The shape of the tyres cause it to slowly but surely migrate up to the surface of the disposal site and thereby disrupt the degradation process. Tyres also resist degradation as they are manufactured to resist both thermal and biological degradation, as well as mechanical wear. Furthermore, tyres resist ultraviolet radiation, ozone and other oxidants, as well as water and ice. Fires is used tyre depots or waste dumps containing tyres are problematic, as they have proven to very difficult to extinguish. In addition, such fires release toxic substances both in the smoke and in the water used to extinguish the fire.
The reuse of vulcanised rubberxe2x80x94a large source of which is used tyresxe2x80x94has posed several problems, in particular how to turn rubber waste into usable raw material for new products. As a result, only a lesser amount of the rubber waste is reused and thus the main part has to be burned, used as landfill or dumped together with municipal waste. The recycling of rubber has so far been focused on relatively uncomplicated applications, such as blasting mats, as an additive in asphalt or in the production of low vibration flooring.
Sheared or shredded rubber fragments can also be used as such, for purposes as improving soil, as filling material for building work, e.g. in road construction for recreation and trotting tracts, etc. Mixing in with compost has also been suggested.
U.S. Pat. No. 4,386,182 states, that the particulate vulcanised rubber for use in manufacturing thermoplastic electrostatic compositions should have a mean particle size of below 1.5 mm. According to another document, U.S. Pat. No. 5,157,082, concerning the manufacture of a thermoplastic composition including particles of vulcanised rubber, the rubber is required to be in the form of ground, small dispersed particles essentially of 1.5 mm number average or below. It is however difficult to achieve this degree of disintegration in industrial applications. The mechanical disintegration or grinding of vulcanised rubber to a particle size of 1.5 mm and below is made very difficult by the elastic properties of the material and the considerable development of heat during the mechanical treatment. Further, the elevated temperatures increases the elasticity of the rubber.
The presently applied solution to the above problem of disintegrating an elastic material to a homogeneous and fine particle size is the so-called cryogenic method. According to this method, rubber articles or rough rubber fragments are cooled to about minus 80xc2x0 C. with liquid nitrogen. At that temperature, the rubber becomes brittle and can be crushed mechanically, typically in a hammer mill. The thus produced particulate rubber is then sieved in shaking sieves and sorted in the desired particle size fractions. The rubber powder is usually sorted in fractions from about 0 to 0.2 mm until 1 to 4 mm. In practice, however, the most desired fractions i.e. those below 1.5 mm, only constitute a lesser part of the total amount of particulate rubber. The above method is further relatively cost intensive, as the processing of 1 kg rubber requires about 0.5 kg nitrogen.
Further, WO 98/20067, by the present inventor, discloses a process for production of a material composition where recycled particulate vulcanised rubber constitutes the main component. The description discloses a process and composition, characterised by the use of expandable microspheres. The expandable microspheres are added in an amount of about 2 to 30 percent per weight of the composition.
Thus there still exists a need for new methods of treating elastic waste and in particular vulcanised rubber, in order to turn it into useful raw material and make possible its efficient reuse.
The present inventor has surprisingly found, that the above problems are solved by a method according to the attached claims. In summary, the problems are overcome by first forming a composite body, consisting mainly of vulcanised rubber waste and a lesser amount of a thermoplastic material as a binder, and then subjecting this composite body to mechanical treatment. A homogeneous, fine particulate powder is produced, more easily and at a lower cost than by the prior art methods.